The present invention relates to a catalyst for removing nitrogen oxides contained in a combustion exhaust gas such as a boiler exhaust gas, especially nitrogen oxides contained in coal combustion exhaust gas.
As a method of removing NOx generated from boilers and various combustion furnaces for the purpose of controlling air pollution, selective catalytic hydrogenation in which a catalyst is used to effect catalytic decomposition into nitrogen and oxygen while adding ammonia gas to an exhaust gas is applied widely to gases, oils and coal.
In the denitration method mentioned above, especially when employing current catalysts applied to the coal combustion exhaust gas, a problem that deposition of calcium and arsenic dusts contained in the exhaust gas causes time course reduction in the catalyst performance is experienced.
The reduction in performance as mentioned above causes disadvantage of frequent replacement of a large amount of catalyst, and accordingly a catalyst having a higher and long lasting performance when compared with the current catalysts is desired.
It is known that the causes for the degradation of the catalysts is the deposition of compounds contained in the dust of the exhaust gas such as calcium and arsenic onto the surface of the catalyst, which results in masking of catalytic active metals and preventing the reactive gases such as NOx and NH.sub.3 from diffusing into the internal area of the catalyst. Since the dust is deposited in a major amount within the depth of about 50 .mu.m from the surface of the catalyst, the durability of the catalyst is considered to be improved by means of coating a component which is worn out gradually as the dust is deposited on the catalyst surface whereby exposing a new surface sequentially.
Accordingly, we invented a two-layered denitration catalyst comprising a lattice catalyst layer as a lower layer covered with a coating as an upper layer having a abrasion resistance against dusts which is lower than that of the lower layer (Unexamined Japanese Patent Application 7-120997). However, since the abrasion resistance of the upper layer coating became extremely low with a certain particle size distribution and the upper layer is worn out instantly when the two-layered catalyst is exposed actually to an exhaust gas, the catalyst suffered from the disadvantage that the dust deposition is not satisfactorily be suppressed. Thus, in such two-layered catalyst, it is important that the abrasion resistance of the upper coating layer should appropriately be high but lower than that of the lower lattice catalyst layer.